This invention relates to restoration of service in a telecommunications network.
With the advent of SONET rings, customer expectation of rapid network restoration has taken a substantial leap. Prior to the optical transport era, failed network connectivity due to a cable cut typically took four to six hours for manual repair. In 1989, AT&T introduced FASTART™ in which a central operations system (called “RAPID”) oversees network connectivity with the aid of a team of monitors strategically placed throughout the network. When a failure occurs at a network element or a facility, alarms from the monitors with a view of the failure are sent to RAPID for root cause analysis. RAPID correlates the failed component to the disabled services, generates a list of service-bearing facilities to be restored, and proceeds with restoration based on a priority ordering of the service facilities. Restoration is effected using dedicated spare capacities that are strategically distributed throughout the network, in amounts averaging about 30% of the service capacity. Typically, the Time-To-Restore metric ranges from three minutes for the first channel restored on up to ten or twenty minutes for the last few channels in large scale failure events. This was a major improvement over the performance of prior restoration paradigms.
Still, FASTAR has certain limitations rooted in its central control architecture. For example, central collection of alarms creates a bottleneck at the central processor. In a large scale failure event, many alarm messages, perhaps from several monitors, need to be sent to the central processor. The central processor must stretch its event window in order to have reasonable assurrance of receiving all messages and obtaining a complete view of the failure. Also, the problem of planning restoration paths for many disparate routes is mathematically complex and quite difficult to solve, leading to restoration reroute solutions that are typically sub-optimal.
In 1995, network elements and transport facilities conforming to the SONET standards were introduced into AT&T transport network. The SONET standards introduced two new topographical configurations, namely, linear chain and closed ring, and in the latter the new restoration paradigm of ring switching. SONET linear chains and rings employ stand-by capacities on a one-for-one basis. That is, for every service channel, there is a dedicated, co-terminated protection channel. As in the older technologies, when a failure occurs on the service line of a span in either a linear chain or a closed ring, the SONET Add/Drop Multiplexers (ADMs) adjacent to the failed span execute a coordinated switch to divert traffic from the failed service channel to the co-terminated protection channel. When both the service and protection lines of a span have failed, however, a SONET ring provides the further capability to switch traffic on the failed span instead to the concatenated protection channels on surviving spans completing a path the opposite way around the ring. The ADMs at the two ends of the failed span each loop the affected traffic back onto the protection channels of the adjacent spans, whence the remaining ADMs around the ring cooperate by completing through connection of the protection channels the entire way around the ring. Since failure detection and protection switching are done automatically by the ADMs, restoration is typically fast and can routinely take less than 200 ms. In short, by setting aside a 100% capacity overhead in the standby mode and configuring facilities in closed rings, SONET standards make possible a three orders of magnitude improvement in restoration time over FASTAR. The challenge has thus shifted to designing a network that is restorable with SONET ring-like performance but without the high penalty in required overhead capacity.